1. Field of the Invention
The present invention relates to a MOS semiconductor device, such as a vertical MOSFET, an insulated gate bipolar transistor (abbreviated as IGBT), or a smart power device, having a main unit element and a sense unit element for monitoring the current in the main unit element.
2. Discussion of the Related Art
When power MOSFETs or IGBTs are incorporated into a power converting device, it may be necessary to monitor the value of the current flowing through the semiconductor element by outputting a sense signal to the exterior of the device in order to protect the semiconductor device and elements from being damaged. FIG. 2 is an equivalent circuit of an IGBT which has the ability to sense an overcurrent flowing through the source-drain path of the IGBT. As shown in FIG. 2, a single semiconductor element 20 contains a plurality of IGBTs, a main cell IGBT element 21 and a sense cell IGBT element 22. Elements 21 and 22 have a common drain terminal D and a common gate terminal G. Main unit element 21 and sense unit element 22 have source terminals S and S', respectively. A sense resistor R is inserted between source terminals S and S'. Load 23 and power source 24 are connected to common drain terminal D. With this arrangement, when a voltage is applied to common gate terminal G, on-currents I and I' flow through main unit element 21 and the sense unit element 22, respectively. On current I is proportional to on-current I' and can be determined from the voltage representative of the product of the resistor R and the on-current I'.
FIG. 3 is a cross sectional view showing cell structures of a main unit element 21 and a sense unit element 22 of an IGBT. In main unit element 21, a p.sup.- base layer 4 (first region) and an n.sup.+ source layer 5 (second region) are formed in a surface region of base layer 4. A p.sup.+ well 6, partially overlapping source layer 5, is also formed in the surface region of the first major surface of an n.sup.- layer 1. The second major surface of n.sup.- layer 1 is formed on n.sup.- buffer layer 2. N.sup.+ buffer layer 2 is formed on p.sup.+ drain layer 3 (fifth region). The sense unit element includes a p.sup.- base layer 41, an n.sup.+ source layer 51 formed in base layer 41, and a p.sup.+ well 61 partially overlapping source layer 51. The region of the base layer 4 located between source layer 5 and n.sup.- layer 1 serves as a channel forming region 7. Similarly, a portion of base layer 41, located between source layer 51 and n.sup.- layer 1, serves as channel forming region 71.
Gate oxide films 8 are formed on the first major surface of the n.sup.- layer 1, and gate electrodes 9 are further formed on the gate oxide films 8. A conductive layer is insulated from gate electrodes 9 by insulation film 10 and divided into source electrodes 11 and 12 connected to source terminals S and S', respectively. Source electrode S' picks up the sense signal. Source electrode 12, having a reduced area, contacts both p.sup.+ well 61 and n.sup.+ source layer 51 through openings in insulation film 10. Source electrode 11 contacts both p.sup.+ well 6 and n.sup.+ source layer 5. P.sup.+ drain layer 3 contacts drain electrode 13, which is connected to the common drain terminal D.
In operation, when a positive potential is applied to common gate terminal G of the IGBT, electrons are generated in channel forming regions 7 and 71 beneath both oxide films 8, thus forming channel inversion layers. N.sup.+ source layers 5 and 51 are electrically connected to n.sup.- layer 1 via the channel inversion layers, so that electrons flow from the n.sup.+ source layers through the channel inversion layers, the n.sup.- layer 1, and the n.sup.+ buffer layer 2 into the p.sup.+ drain layer 3. In conjunction with the flow of electrons, holes are injected from p.sup.+ drain layer 3 into n.sup.- layer 1 through n.sup.+ buffer layer 2, so that conductivity modulation occurs in the n.sup.- layer 1 so that the resistance in this region is reduced. The low on-resistance facilitates current flow between drain electrode 13 and source electrode 11, and between the drain electrode and sense-signal-pickup source electrode 12. These currents are proportional to the number of cell structures which are formed in the regions of the main unit element and the sense unit element, respectively.
The semiconductor device described above has the following problem. In order to output a sense signal from the IGBT, a metal wire is bonded to the surface of the source electrode 12 by an appropriate bonding technique. Wire bonding, however, requires relatively wide area of 0.5 to 1 mm.sup.2. Since the cell coming in contact with source electrode 11 of the main unit element cannot be formed under the source electrode 12, in the conventional structure, the amount of current flowing into sense-signal pickup source electrode 12 is increased and the current flowing into source electrode 11 of the main unit element 21 is correspondingly decreased. As a result, the power loss through resistor R in device is increased.
The semiconductor device described above has another problem. When the semiconductor device is in the on-state, depletion layers are formed which extend from the junctions of p.sup.- base layers 4 and 41 and the n.sup.- layer 1 into a portion of n.sup.- layer 1 between p.sup.- base layers 4 and 41 and the p.sup.+ wells 6 and 61. These depletion layers extend into regions beneath source electrode 12 which do not contain any cell structure. When the semiconductor device is turned on and off, the voltage applied between source electrodes 11 and 12 and drain electrode 13 varies, causing these depletion layers to repeatedly appear and disappear. The current that flows into source electrode 12 due to the compensating charge or discharged charge (i.e., stored charge at these switched p-n junctions) resulting from the repetitive appearance and disappearance of the depletion layer is not proportional to the current that flows into the source electrode 11. As a result, as shown in FIG. 4, a transient response region 40 (dotted line) occurs in which the relationship between the sense signal current I' and the main current I, when the semiconductor device in an on state, is nonlinear. When this occurs, the sense current signal I' cannot be used to accurately monitor the main current I.